A. Field of Invention
The present invention relates generally to devices for reducing vibration of a vibrating structure and more particularly to a device for damping vibration of a control system structure.
B. Description of Related Art
It is often necessary or desirable for an operator of equipment to manually and directly control and manipulate a vibrating structure, for example, when the power source causes vibration of equipment that must be controlled manually. The vibration communicated to the operator may be undesirably tiring or uncomfortable or interfere with the control by the operator. Traditional methods of attempting to control vibration include using traditional hydraulic damping mechanisms to reduce the vibration and isolating the operator from the source of vibration to reduce the effect of the vibration. However, traditional hydraulic damping mechanisms are generally relatively heavy and cumbersome and are not desirable for some applications. The method of isolation of the operator from direct connection to the source of vibration may undesirably eliminate some of the control precision and the sensory feedback to the operator that is a beneficial aspect of direct control.
An example of the effect of the communication of vibration to the operator is found in small helicopters. Vibration in helicopter cyclic control systems is frequently caused by the mechanical linkage of the control system with the swash plate which is linked to the main rotor. To control flight direction and attitude, the operator manipulates the cyclic control system to tilt the swash plate, side to side, fore and aft, and any combination thereof. The swash plate also progressively changes the angle of attack of the rotor blades. The advancing rotor blade has a very small angle of attack when in forward flight because the relative wind plus rotor speed results in great lift. However, the retreating blade, has low relative airspeed and must have a large angle of attack to give an equal lift, else the helicopter would not fly level nor straight. For this reason, the pitch of each rotor blade must be rapidly changed twice in each complete rotation. The rapid change of pitch of the rotor blade is frequently accomplished by means of direct mechanical interaction between the swash plate and a rotor pitch controlling arm. The interaction between the swash plate and each rotor pitch controlling arm is rapid and forceful and occurs with each revolution of the rotor. The force of the change of rotor pitch is communicated to the swash plate and causes vibration in the cyclic control system that the pilot uses for flight. At no phase of any flight may the pilot remove his hand from the cyclic control, and therefore, vibration of the cyclic control is tiring and unpleasant for the pilot during long flights.
In large helicopters, costly and complex hydraulic power controls assist the cyclic control system and serve to isolate the pilot from vibration and may also provide hydraulic vibration damping. Smaller helicopters may use weights to absorb the shake; however, the smallest helicopters have no vibration dampening and the vibrations are absorbed by the hand of the pilot. Common vibration damper mechanisms are heavy and could not easily be tuned to the best frequency for optimum results. The need is for a relatively light weight, inexpensive and effective mechanism for reducing vibration experienced by the helicopter pilot from the cyclic control system.